Implemented CHIP Certificate (chip-cert) Tool. (#6403)

chip-cert tool provides command line interface (CLI) utility used
for generating and manipulating CHIP certificates and CHIP
private keys material.

chip-tool supports the following commands:
  - generate CHIP certificate
  - convert CHIP certificate format
  - convert CHIP private key format
  - validate CHIP certificate chain
  - resign CHIP certificate
  - print CHIP certificate

BUILD.gn

@@ -30,6 +30,8 @@ assert(chip_root == "//")
 import("${chip_root}/build/chip/tests.gni")
 import("${chip_root}/build/chip/tools.gni")
 
+import("//src/crypto/crypto.gni")
+
 if (current_toolchain != "${dir_pw_toolchain}/dummy:dummy") {
   declare_args() {
     chip_enable_python_modules =
@@ -107,6 +109,9 @@ if (current_toolchain != "${dir_pw_toolchain}/dummy:dummy") {
         "${chip_root}/src/qrcodetool",
         "${chip_root}/src/setup_payload",
       ]
+      if (chip_crypto == "openssl") {
+        deps += [ "${chip_root}/src/tools/chip-cert" ]
+      }
       if (chip_enable_python_modules) {
         deps += [ "${chip_root}/src/controller/python" ]
       }

src/credentials/CHIPCert.h

@@ -260,7 +260,6 @@ class ChipDN
      **/
     bool IsEmpty() const { return RDNCount() == 0; }
 
-protected:
     ChipRDN rdn[CHIP_CONFIG_CERT_MAX_RDN_ATTRIBUTES];
 
     uint8_t RDNCount() const;

src/tools/chip-cert/BUILD.gn

@@ -0,0 +1,54 @@
+# Copyright (c) 2021 Project CHIP Authors
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import("//build_overrides/chip.gni")
+
+import("${chip_root}/build/chip/tools.gni")
+
+import("//build/config/linux/pkg_config.gni")
+
+# TODO: Move it into common BUILD file to avoid multiple runs of pkg_config
+pkg_config("openssl_config") {
+  packages = [ "openssl" ]
+}
+
+assert(chip_build_tools)
+
+executable("chip-cert") {
+  sources = [
+    "CertUtils.cpp",
+    "Cmd_ConvertCert.cpp",
+    "Cmd_ConvertKey.cpp",
+    "Cmd_GenCert.cpp",
+    "Cmd_PrintCert.cpp",
+    "Cmd_ResignCert.cpp",
+    "Cmd_ValidateCert.cpp",
+    "GeneralUtils.cpp",
+    "KeyUtils.cpp",
+    "chip-cert.cpp",
+    "chip-cert.h",
+  ]
+
+  public_configs = [ ":openssl_config" ]
+
+  cflags = [ "-Wconversion" ]
+
+  public_deps = [
+    "${chip_root}/src/credentials",
+    "${chip_root}/src/crypto",
+    "${chip_root}/src/lib/asn1",
+    "${chip_root}/src/lib/core",
+    "${chip_root}/src/lib/support",
+  ]
+}